Romaine lettuce should provide a product at harvestable stage, which is accepted by processing industry and/or consumers. Therefore the harvestable product should not have tipburn, and it should have a dark green outer leaf color. Further it should be sufficiently headed, and as a result of this heading provide sufficient yellow-colored heartleaves.
Romaine lettuce is brought to the market in the form of wholehead, hearts, single leaves, or cut leaf pieces. For the purpose of wholehead and heart production it is very important that the leaves are wide enough to overlap and maintain head integrity. It also important that core internodes and petioles are short enough to give a closed base. The closed base and overlapping head leaves are reducing the product surface resulting in less wilting and a longer shelflife. This is desired by traders, retailers and consumers.
Lettuce can be attacked by many diseases and pests, such as downy mildew (Bremia lactucae) and aphids. These problems have to be controlled by growers at high costs. Especially the currant-lettuce aphid Nasonovia ribisnigri is causing problems in lettuce, as it has the unique feature of multiplying deeply hidden in the heart of the lettuce plant. In the heart it cannot be reached by most insecticide treatments against aphids. Pesticides are used to control against these pests and diseases and they form a risk for the grower using them, as well as their residues are reducing the confidence of the potential consumers in the healthiness of the product.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.